Integrated circuits frequently contain digital-to-analog converters which convert a digital input value to an analog output signal. In many applications, it is preferred that the conversion be linear, that is, that the analog output change proportionately to a change in the digital input. However, various factors can introduce nonlinearities in the conversion operation. For example, variations in the fabrication processes from one integrated circuit die or wafer to the next can cause variations in the performance of the analog circuits disposed on those dies or wafers. As the scaling of integrated circuits continues to shrink, it is believed that such variations in performance may increase.
Various tests have been devised to detect performance variations during the manufacture of integrated circuit devices. For example, FIG. 1 shows an integrated circuit 100 on a die 110. The integrated circuit includes a digital-to-analog converter 120 having an analog output 125. To test the digital-to-analog converter 120, the integrated circuit 100 may further include a step function generator 130 which generates a sequence of digital input codes. Alternatively, a sequence of digital input codes may be scanned into the die 110 through an external input port 135.
The value of each digital input code is typically incremented (or decremented) a fixed amount each step by the function generator 130. If the digital-to-analog conversion is linear, the value of the analog output of the digital-to-analog converter 120 will rise (or fall) a fixed amount as the digital input code is incremented (or decremented) a fixed amount each step.
To test the linearity of the conversion by the digital-to-analog converter 120, the integrated circuit 100 may further include an analog-to-digital converter 140 which is on board the same die 110 as the digital-to-analog converter 120. The analog-to-digital converter 140 converts the analog output of the digital-to-analog converter 120 to a digital output code, which is typically at a higher resolution that the input to the digital-to-analog converter 120, and is output at an external output port 150 of the die 110. The digital output codes produced by the analog-to-digital converter 140 in response to each step of the step function generator 130, can be collected by various devices external to the die 110 such as a general purpose computer or dedicated testing apparatus coupled to the external output port 150 of the die 110. Frequently, a digital output code is scanned out of the die at each step of the function generator 130. Such a process can be time consuming. The digital data collected is typically processed by external apparatus to determine the linearity and other characteristics of the conversion process. In other prior devices, an analog-to-digital converter 140 may also be external to the die 110.
FIG. 2 shows another architecture for testing a digital-to-analog converter 120a. In this approach, the analog output is provided at an external analog port 125a of the die 110a. Consequently, the analog output voltage of the digital-to-analog converter 120a may be measured directly by suitable measurement instrumentation external to the die 110a. Again, an analog output voltage of the die 110a may be measured or otherwise detected at each step of the function generator 130a. These measurements of the analog output signals produced by the digital-to-analog converter 120a in response to each step of the step function generator 130a, can be collected by various devices external to the die 110 such as a general purpose computer or dedicated testing apparatus coupled to the external output port 125a of the die 110a. Here too, the analog data collected is typically processed by such apparatus to determine the linearity of the conversion process.